In more and more high-standard industrial automation applications, technological progress is changing the performance-cost ratio between stepping motor and servo motor.
After adopting the closed-loop technology, the closed-loop stepping motor provides users with excellent accuracy and efficiency, which can not only achieve the performance of servo motor, but also have the advantage of low price of stepping motor. The lower-cost stepping motor is gradually penetrating into the application fields that were originally dominated by high-cost servo motors.
Comparison between stepping motor and servo motor
According to the traditional concept, the servo control system has better performance in applications that require speed exceeding 800 RPM and high dynamic response. Stepping motor is more suitable for applications with low speed, low to medium acceleration and high holding torque.
So what is the basis of this traditional concept about stepping motor and servo motor?
Let’s analyze it in detail below.
The stepping motor rotates in a step-by-step manner, and a magnet is gradually pulled by a magnetic coil to reach the next position from one position. To make the motor move 100 positions in any direction, the circuit needs to step the motor 100 times. Stepping motor uses pulse to realize incremental movement, which can realize accurate positioning without using any feedback sensor.
The movement methods of the servo motors are different. It connects a position sensor–encoder–to the magnetic rotor to continuously detect the accurate position of the motor. The servo will monitor the difference between the actual position of the motor and the commanded position, and adjust the current accordingly. This closed-loop system can keep the motor in the correct motion state.
2. Simplicity and cost
Stepping motor is not only cheaper than servo motor, but also easier to debug and maintain. Stepping motor is stable at rest, and can keep its position (even with dynamic load). However, if some applications have higher performance requirements, more expensive and complex servo motors must be adopted.
There are important differences between stepping motor and servo motor in applications that need to know the exact position of the machine at any time. In the open-loop motion application controlled by stepping motor, the control system thinks that the motor is always in the correct motion state. However, after encountering problems, such as the motor stalling due to a stuck part, the controller can’t know the actual position of the machine, which leads to the loss of position. The closed-loop system of the motor itself has an advantage: if it gets stuck by an object, it will be detected immediately. The machine will stop operating and never lose its position.
4. Speed and torque
The performance difference between stepping motor and servo motor comes from their different motor design schemes. The number of poles of stepping motor is much more than that of servo motor. Therefore, when stepping motor rotates for a whole turn, it needs much more times of winding current exchange, which leads to the rapid decrease of its torque when the speed increases. In addition, if the maximum torque is reached, the stepping motor may lose its speed synchronization function. For these reasons, servo motor is the first choice in most high-speed applications. On the contrary, stepping motors with more poles have advantages at low speed, because stepping motors have torque advantages compared with servo motors of the same size.
With the increase of speed, the torque of stepping motor will decrease.
5. Heat and energy consumption
Open-loop stepping motor uses fixed current, and will emit a lot of heat. The closed-loop control only provides the current required by the speed loop, thus avoiding the problem of motor heating.
Servo system is most suitable for high-speed applications involving dynamic load changes, such as mechanical arm. Stepping control system is more suitable for applications that require low to medium acceleration and high holding torque, such as 3D printers, conveyor belts, countershafts, etc. Because the cost of stepping motor is lower, the cost of automation system can be reduced after it is used. If the motion control system needs to make use of the characteristics of servo motors, it must prove that these high-cost motors are worth the money.
Driven by the progress of closed-loop technology, stepping motors can penetrate into the high-performance and high-speed application fields that used to belong to servo motors.
Closed-loop stepping motor has changed the performance-cost ratio in many motion control applications. Because of its excellent accuracy and energy efficiency, stepping motors can be used in fields where more expensive servo motors were the main ones in the past. The success of the progressive closed-loop motor also shows us a possibility-low-cost stepping motor to replace high-cost servo motor.